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are a class (Arachnida) of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek word ἀράχνη (aráchnē), meaning "spider".

Almost all extant arachnids are terrestrial. However, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well. They comprise over 100,000 named species, including spiders, scorpions, harvestmen, ticks, mites and Solifugae.

Arachnophobia

Arachnophobia or arachnephobia (from the Greek: ἀράχνη, aráchnē, "spider" and φόβος, phóbos, "fear") is a specific phobia, the fear of spiders and other arachnids such as scorpions. It is a manifestation of zoophobia, among the most common of all phobias.[1] The reactions of arachnophobics often seem irrational to others (and sometimes to the sufferers themselves). People with arachnophobia tend to feel uneasy in any area they believe could harbor spiders or that has visible signs of their presence, such as webs. If arachnophobics see a spider they may not enter the general vicinity until they have overcome the panic attack that is often associated with their phobia. In some extreme cases, even a picture or a realistic drawing of a spider can also trigger fear. Arachnophobics may feel humiliated if such episodes occur in a social setting.

SPIDERS

Spiders have 8 legs that are jointed and some have fangs that inject venom to their prey. There are approximately 40,000 species of spiders. They are air breathing and they can live in any habitat with the exception of air and sea colonization. A vegetarian species was discovered in 2008 but all others are known predators. They feed upon small insects (flies etc.) They spin a web to catch their victims and the female weave silk egg cases that may contain hundreds of eggs.

SCORPIONS

Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger.

Scorpions are found widely distributed over all continents, except Antarctica, in a variety of terrestrial habitats except the high latitude tundra. Scorpions number about 1752 described species, with thirteen extant families recognised to date. The taxonomy has undergone changes and is likely to change further, as a number of genetic studies are bringing forth new information.

Though the scorpion has a fearsome reputation as venomous, only about 25 species have venom capable of killing a human being.

SCHIZOMIDA

Schizomida (common name shorttailed whipscorpion) is an order of arachnids, superficially resembling spiders and generally less than 5 millimetres (0.20 in) in length.

The order is not yet widely studied. As of 2005, more than 230 species of schizomids have been described worldwide, most belonging to the Hubbardiidae family. A systematic review including a full catalogue may be found in Reddell & Cokendolpher

Female schizomida

Thelyphonida

Thelyphonida is an arachnid order comprising invertebrates commonly known as vinegaroons (or vinegarroons). They are often called uropygids in the scientific community after the former order Uropygi (which originally also included the order Schizomida). They are also known as whip scorpions because of their resemblance to true scorpions and because of their whiplike tails.

Solifugae

Solifugae is an order of Arachnida, known as camel spiders, wind scorpions or sun spiders, comprising more than 1,000 described species in about 153 genera. They may grow to a length of 12 cm (4.7 in), including legs, and have a body comprising an opisthosoma (abdomen) and a prosoma (head) with conspicuously large chelicerae, which are also used for stridulation. Most species live in deserts and feed opportunistically on ground-dwelling arthropods and other animals. A number of urban legends exaggerate the size and speed of Solifugae, and their potential danger to humans.

Acari

Acari (or Acarina) are a taxon of arachnids that contains mites and ticks. The diversity of the Acari is extraordinary and its fossil history goes back to at least the early Devonian period. As a result, acarologists (the people who study mites and ticks) have proposed a complex set of taxonomic ranks to classify mites. In most modern treatments, the Acari is considered a subclass of Arachnida and is composed of 2-3 superorders or orders: Acariformes (or Actinotrichida), Parasitiformes (or Anactinotrichida), and Opilioacariformes; the latter is often considered a subgroup within the Parasitiformes. The monophyly of the Acari is open to debate, and the relationships of the acarines to other arachnids is not at all clear. In older treatments, the subgroups of the Acarina were placed at order rank, but as their own subdivisions have become better-understood, it is more usual to treat them at superorder rank.

Most acarines are minute to small (e.g. 0.08–1.00 millimetre or 0.0031–0.039 inch), but the largest Acari (some ticks and red velvet mites) may reach lengths of 10–20 millimetres (0.39–0.79 in). It is estimated that over 50,000 species have been described (as of 1999) and that a million or more species are currently living. The study of mites and ticks is called acarology (from Greek ἀκάρι, akari, a type of mite; and -λογία, -logia)